Trypan Blue Controls

I work in a clinical lab where we process bone marrow, cord blood and PB stem cells for transplant. We use TB for cell viability (and also 7-AAD). We are looking for some sort of control for our trypan blue. We need something to prove the trypan blue is working correclty, perferably something we can make in house. Any ideas? We were hoping something that could kill half the cells and leave half to show both positive and negative??